October 3, 2002 Thomas Waszak, CISSP Black Hat Briefing Asia 2002.
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Biogas as a Residential Energy Source
For South Asia
Daniel Thomas
ME 5080 Term research paperDecember 11, 2003
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Biogas as a Residential Energy Source for South Asia Daniel Thomas, 1
Abst rac t
Because of high population growth and high growth trends in energy consumption, South
Asia will become increasingly important in global energy consumption. The high proportion of
residential energy use for cooking and current high reliance on biomass give sustainable biogas
energy great potential in this region. A first and second law analysis of biogas use, as applied to
domestic cooking, was conducted. 11, 380 kJ of captured cooking heat can be obtained from one
livestock unit per day by digesting the manure and burning the biogas, since the second law
effectiveness of this process is about 47%. Only 9580 kJ/LU-day is obtained from burning the
same quantity of dried manure. In addition, biogas can be generated from other organic feed
stocks and produces an effluent with high fertilizer value in addition to the methane-rich gas.
When other benefits such as financial savings, public health, nutrient retention, time savings, and
convenience are considered, biogas becomes an even more viable energy source for South Asia.
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Biogas as a Residential Energy Source for South Asia Daniel Thomas, 2
I n t roduc t ion t o reg iona l energy use
South Asia is usually defined as the five nations that comprise the Indian subcontinent:
India, Pakistan, Bangladesh, Nepal and Sri Lanka (Figure 1). This is an area with a rapidly
growing population and energy consumption. Figure 2 shows that the population of the region,
already at 22%, is increasing as a fraction of the world’s population.
Figure 1: Nations of South Asia, with current
population in millions.1
2000 World Population
Africa, 784.3
South Asia, 1342Rest of Asia,
2349.5
USA, 278.4
Rest of
Americas, 550.3 Europe, 729.8
Pacific, 31.2
2025 World Population
Pacific, 40.8
Europe, 703.4
Rest of
Americas, 734.6USA, 325.6
Rest of Asia,
2897.6South Asia,
1833.8
Africa, 1298.2
Figure 2: 2000 and 2025 population of South Asia as
a fraction of world population.2
The energy consumption of South Asia is also increasing rapidly, as demonstrated by
Figure 3. This trend is even more evident in Figure 4, where the normalized growth trends show
that the Middle East, Asia and Oceana are the regions with the largest growth rates in energy
consumption. Looking into this region in more detail, Figure 5 shows that the large countries of
South Asia have energy consumption growth trends significantly steeper than those of the larger
Asia and Oceana region.
The large increases in population stimulate large increases in energy consumption, since
a large proportion of the energy is consumed by the residential sector (Figure 6). With less
developed industry and commercial sectors, much of the consumption is of energy suitable for
household cooking and agriculture. This is reflected in Figure 7, which shows that the heavy
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Biogas as a Residential Energy Source for South Asia Daniel Thomas, 3
reliance on biomass. Almost 90% of Nepal’s energy comes from biomass, which is a stark
contrast to the 4% shown on the USA series, included for comparison. India has large coal
resources and more industry than the other countries shown, but still relies on biomass for 18%
of its total energy consumption. These patterns are typical of much of the rest of Asia and Africa,
so conclusions reached for South Asia may well be equally applicable in these other regions.
0
20
40
60
80
100
120
140
1992 1993 1994 1995 1996 1997 1998 1999 2000 2001
Year
C o n s u m
p t i o n ( Q u a d s )
North America
South and Central Am.
Western Europe
Eastern Europe
Middle EastAfrica
Asia & Oceana
Figure 3: Primary energy consumption by region, 1992 – 2001.3
0.5
0.6
0.7
0.8
0.9
1
1992 1993 1994 1995 1996 1997 1998 1999 2000 2001
Year
F r a
c t i o n o f M a x . C o n s u m p t i o n
North America
South and Central Am.
Western Europe
Eastern Europe
Middle East
Africa
Asia & Oceana
Figure 4: Normalized energy consumption by region, 1992 – 2001.4
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Biogas as a Residential Energy Source for South Asia Daniel Thomas, 4
0.5
0.6
0.7
0.8
0.9
1
1992 1993 1994 1995 1996 1997 1998 1999 2000 2001
Year
F r a c t i o n o f M a x . C o
n s u m p t i o n
India
Pakistan
Bangladesh
United States
Figure 5: Normalized energy consumption by country, 1992 – 20015
0%
20%
40%
60%
80%
100%
P e r c e n t
a g e o f t o t a l e n e r g y r e q u i r e m e n t
India Pakistan Bangladesh Nepal Sri Lanka USA
Country
Industry/ Construction Transportation Household/ Agriculture
Energy Convers ion Losses
Figure 6: Energy end-use by sector.6
0%
20%
40%
60%
80%
100%
P e r c e n t a g e
o f t o t a l e n e r g y r e q u i r e m e n t
India Pakistan Bangladesh Nepal Sri Lanka USA
Country
Coal Oil Gas Biomass Other
Figure 7: Energy sources for South Asia countries.7
The countries of South Asia and other developing nations have rapidly increasing energy
demands but are also those that will be most negatively affected by global warming. There hastherefore been considerable interest in moving to sustainable energy sources in their early growth
phase. Because of the high cost involved in renewable energy technologies such as wind and
photovoltaic solar, biomass is a very viable solution, if it can be used in a sustainable way.
(Deforestation, nutrient loss and smoke are major concerns.)
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Biogas as a Residential Energy Source for South Asia Daniel Thomas, 5
I n t roduc t ion t o b iogas
Biogas consists mostly of methane, and is the product of anaerobic digestion of organic
material. The production of biogas is a natural process, but it has only been harnessed for energy
production since about 1950. The basic process is shown in Figure 8, and can use crop wastes,
livestock manure, human excrement and urine, by-products of agricultural industries, forest litter
or aquatic wastes and weeds as feedstock. This can be a continuous process, with wastes fed
through one port to the digester and slurry exiting at another. The main requirement of the
digester is that it be airtight so that oxygen does not enter and the biogas is trapped for use. Many
designs have been developed, the most common of which are the fixed dome (Figure 9), floating
dome (Figure 10) and polyethylene bag (Figure 11) digesters.
Figure 8: Basic biogas production process with main product uses.8
Figure 9: Fixed dome bio-digester design developed
and used extensively in China.
Figure 10: Floating dome bio-digester design
developed and used extensively in India
Methane
Generator
Animal WastesNight Soil
Crop ResidueDomestic Waste
Fuel
Sludge
Mechanical Energy
Heat and Light
Electrical Energy
Nutrients returned to soil
Improved soil structure
Increase in organic matter
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Biogas as a Residential Energy Source for South Asia Daniel Thomas, 6
Figure 11: Short version of the polyethylene bag biogas digester design, shown installed in an earthen ditch.
Eff ic ienc y Analys is o f b iogas energy
In calculating a first-law efficiency for biogas use, it is interesting to compare the
efficiency of using biogas generated from a given quantity of feedstock to the efficiency of direct
combustion of the same material. For an example, cooking was chosen since it is the largest end-
use in most of South Asia. Cattle manure was chosen as the fuel since it is a very widely used
fuel and often the main alternative to fuel wood (the use of which leads to deforestation).
A 500 kg head of cattle (= 1 Livestock Unit, LU) produces about 40 kg manure per day,
as shown in Figure 12. Although cattle produce more manure per livestock unit than other
animals, cattle manure has a higher moisture content, so the amount of volatile solids is about
equal to that for other livestock. Figure 13 shows the composition of manure for several types of
livestock, the majority being carbohydrates of easy (E) or slow (S) digestibility. The typical
composition of cattle manure is given in more detail in Table 1.
3.1
35.4
3.9
37.8
2.0
26.4
4.3
15.7
5.3
26.0
4.0
24.0
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
P r o d u c t i o
n p e r L i v e s t o c k U n i t ( k g / L U )
Dairy
cattle
Beef
cattle
Swine Sheep Poultry Horses
VS Water
Figure 12: Daily manure production for various
livestock, showing proportion of solids to liquids.9
Figure 13: Composition of manure for various
livestock.10
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Biogas as a Residential Energy Source for South Asia Daniel Thomas, 7
Table 1: Composition of cattle manure on a mass
basis.11
Component Characteristic Mass
composition fraction
Lignin C6H10O5 12%
Carbohydrates (S) C6H10O5 19%
Carbohydrates (E) C6H10O5 43%
Lipid C57H104O6 7%
Protein C5H7O2N 16%
Volatile Fatty Acids C2H4O2 3%
60.0
36.0
2.0
2.0
72.6
11.9
14.5
1.0
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Mass basis Molar basis
CH4 CO2 H2 N2
Figure 14: Typical Composition of Biogas on mass andmolar basis.
The anaerobic digestion of all of these components can be considered with the basic
reaction:
The yield of methane from this reaction can thus be calculated with Bushwell’s formula:12
If all the volatile solids (using composition given in Table 1) were consumed to produce methane
in this reaction, the theoretical yield of methane and biogas (using the composition given in
Figure 14) would be:
But the lignin, slowly digestible carbohydrates, and volatile fatty acids are not digested in a
typical biogas digester retention period (< 3 months), so the ultimate methane yield from the
same manure would be as follows. The volume of biogas is again found using the typical biogas
composition in Figure 14.
42248248224
CH ban
COban
O H ba
nO H C ban
−++
+−→
−−+
VSCH Th kgm
ban
banY /
1612
4.22)4 / 8 / 2 / ( 3
4
++
⋅−+=
VS BG
VSCH
manure
Th
VFA
ThVFA
protein
Th protein
lipid
Thlipid
carb
Thcarb
carb
Thcarb
lignin
Thlignin
total
Th
kgm
kgm
Y
Y yY yY yY yY yY yY E S
/ 783.0
/ 470.0
)370.0(03.)496.0(16.)014.1(07.)415.0(43.)415.0(19.)415.0(12.
3
3
.
4
=
=
+++++=
+++++=
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Biogas as a Residential Energy Source for South Asia Daniel Thomas, 8
0.210.24 0.26 0.24
0.22
0.0630.087 0.087 0.091 0.081
0
0.05
0.1
0.15
0.2
0.25
0.3
A c t u a l B i o g a s Y e
i l d ( m 3 / k g )
M a n u r e
w i t h c a t t l e u r i n e
w i t h s u g a r , u r e a
w i t h s u g a r , l i m e
w i t h l e a v e s
24 days digestion 80 days digestion
Figure 15: Actual production of biogas for various
manure mixtures after 24 and 80 days digestion.13
Figure 16: Typical efficiencies and capital cost of
stoves commonly used in South Asia.14
However, lower retention times result in actual biogas yields being less than half of this ultimate
yield (Figure 15). Variations in feedstock composition can also play an important role in
determining this final yield. Once the biogas yield of manure has been calculated, the energy
produced by the combustion of biogas is found using the following balanced combustion
equation, based on values shown in Figure 14.
VS BG
VS BG
VSCH
manure
U
protein
Th protein
lipid
Thlipid
carb
Thcarb
total
U
kgkmol
kgm
kgm
Y
Y yY yY yY E
/ 0100.0
/ 547.0
/ 329.0
)496.0(16.)014.1(07.)415.0(43.
3
3
.
4
=
=
=
++=
++=
222222224 012.3597.1845.)76.3(7985.145.010.119.726. N O H CO N O H N COCH ++→+++++
VS
VS BG BG
BG BGmanure
P e f e R i f i
kgkJ
kgkmolkmolkJ
kmolkJ kmolkJ E
hnhn E
/ 177,6
) / 010.0( / 541,617
) / 711,718() / 170,101(
,,
==
−−−=
∆−∆=
∑∑°°
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Biogas as a Residential Energy Source for South Asia Daniel Thomas, 9
In cooking the efficiency of the stove in using this energy of combustion varies considerably.
Gas stoves are much more efficient than typical biomass stoves, with efficiencies between 55%
and 60% (Figure 16). This gives a useful heat value of:
With a typical output of 3.1 kgvs /LU·day for cattle (Figure 12), this becomes:
Table 2 shows other typical uses for this same energy output. All across South Asia, manure is
collected and dried for burning, so it is also interesting to compare this output to that given by
simply burning the same quantity of manure. Because of the complex structure of the
hydrocarbons involved, the energy from combustion of the manure is best found from the higher
heating value, calculated using the Dulong formula:15
Tillman gives the following equation for finding the lower heating value from the HHV:16
Table 2: Other uses of calculated energy available
from biogas produced daily by 1 LU.17
Use Quantity
Daily cooking 2.3 persons
Single mantle lamp 10.7 hours
2 hp engine 1.7 hours
24 h refrigeration 1.1 ft3
24 h incubation 2.1 ft3
0
20000
40000
60000
80000
k J / k g o r k J / L U - d a y
Direct combustion Biogas combustion
Qreaction Quseful
Figure 17: Total and useful energy produced by
burning manure directly or the biogas derived from it.
VS
VS
stoveuseful
kgkJ
kgkJ
E Q
/ 3670
/ 611760.0
=⋅=
=η
day LU kJ Quseful ⋅= / 377,11
VS
S H C lb Btu
lb Btu
mmm HHV
/ 517,25
066.06100048.014490
55506100014490 /
=⋅+⋅=++=
[ ]
day LU kJ
kgkJ
lb Btu
kgkJ LHV
m MC HHV LHV
VS
kgKJ
H f lb Btulb Btu
⋅==
⋅⋅⋅+⋅−=
+−=
/ 690,73
/ 770,23
/
/ 326.2)1050065.0910.01050(10970
)105091050(
/
/ /
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Biogas as a Residential Energy Source for South Asia Daniel Thomas, 10
A typical traditional stove in which the manure would be burnt only has an efficiency of 13%,
giving a final useful energy of:
These results are shown graphically in Figure 17. Although much more heat is produced by just
burning the manure, the lower stove efficiencies cause the final useful heat to be lower than if the
same amount of manure had been used to produce biogas which was used for the cooking.
In this case the first law analysis allows the most useful comparison between different
uses for the original biomass fuel. A second law analysis using the availability of the manure
would be difficult, due to the complicated organic components; the standard chemical
availability of proteins, carbohydrates or lipids is not available. However, a useful second law
analysis of the biogas combustion can be done beginning with the balanced combustion
equation:
The adiabatic flame temperature for the reaction is found by satisfying the following equation:
However, the actual flame temperature is usually significantly lower, as confirmed by Gatade
and Rao18
, and can be found using Gaydon and Wolfhard’s correction factor:19
The chemical availability is as follows, using the standard chemical availability of the methane
and hydrogen components of the biogas:
This gives a second law efficiency of:
day LU kJ
day LU kJ
LHV Q stoveuseful
⋅=⋅⋅=
=
/ 580,9
/ 690,7313.0
η
222222224 012.3597.1845.)76.3(7985.145.010.119.726. N O H CO N O H N COCH ++→+++++
K T
hnhnhn
ad
P
e f e
R
i f i
P
ee
1981
,,
=⇒
∆−∆=∆ ∑∑∑ °°
C K
T T ad actual
°===
11121585
)8.0(
ch
f
useful R
a
QT T ) / 1( 0−=ε
BG
BG
chi
i
ich
f
kmolkJ
kmolkJ
ana
/ 251,637
/ 382,235145.0745,830726.0
=⋅+⋅=
=∑
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Biogas as a Residential Energy Source for South Asia Daniel Thomas, 11
This second law efficiency would be useful in improving the efficiency of the biogas stoves, or
other uses of the gas (Table 2).
Advantages and d isadvantages of b iogas energy
Economics: Since biogas digesters currently being developed and used in South Asia are largely
for rural, low-income family cooking the economic benefits of the energy source are very
important. The initial capital cost is high, but this has been lowered by government subsidy and
micro-loan programs.
In a field study of four 3 m3 /day biogas digesters operating in rural India, Pande found all
produced net annual savings of Rs. 1000 – 3000.20 The plant shown in Table 3 was financed
partially by government subsidy but mostly by the owner through initial payment and loan. This
analysis accounts for regular maintenance as well as eventual replacement of parts and
construction repair. Table 4 shows another analysis which shows similar net savings in situations
in which the manure was previously used for either fuel or fertilizer. If this plant were financed
with 25% government subsidy and a loan paid in five installments, net savings would be about
Rs. 500 for the first five years and about Rs. 1500 after repaying the loan.
Table 3: Typical costs and savings of a 4 m3 /day biogas digester in rural India.21
CONSTRUCTION: OPERATION:
Construction cost: 4,980.00 Annual Expenses 2,867.00
Bricks (4000) 1,920.00 Manure (36 tons) 2,000.00
Cement (30 bags) 1,440.00 Interest on loan (12%) 357.00
Brick ballast 250.00 Depreciation on equipment (10%) 64.00
Sand (100 ft3) 150.00 Depreciation on construction (7%) 334.00
Morang (80 ft3) 320.00 Maintenance 112.00
Labour 900.00 Gross Annual Savings 3,640.00
Other costs: 641.00 Annual fuel and lighting expenses 2,040.00
Pipe (20 ft) 160.00 Value of improved manure (20 tons) 1,600.00
Sockets and valves 62.00 Net Annual Savings 1,171.00Paint (2 l) 44.00 Gross savings - expenses 773.00
Stove 250.00 Add back depreciation 398.00
Lamp 125.00
Total 5,621.00 Average Net savings while repaying loan 871.00
Initial payment by owner 621.00 (first 7 years)
Government subsidy 2,140.00 Net savings after repaying loan 1,528.00
Loan 2,860.00 (after 7 years)
%1.47
/ 251,637
/ 541,617)6.0()1585 / 3001(
=
−=
BG
BG
kmolkJ
kmolkJ K K ε
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Biogas as a Residential Energy Source for South Asia Daniel Thomas, 12
Table 4: Cost and Income analysis of a 6 m3 /day biogas plant.
22
Operation: In a study of 97,000 biogas plants installed in India by the non-government
organizations Action for Food Production and Canadian Hunger Foundation, 87% of the plants
were still operating after about 10 years, although many were running inefficiently.23
Much of
the inefficient operation was due to not feeding the digester sufficiently, as were about 30% of
the non-functioning plants. However, as shown in Figure 18 defects in plant and stove
components were also major problems. Problems with defective components also accounts for
the low rate of operation for plants installed over 10 years before the study (Figure 19).
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Biogas as a Residential Energy Source for South Asia Daniel Thomas, 13
Social Problems: loss of cat tle, moving of cattle, f amily dispute, etc.
Technical Problems: pipelin leak, broken dome valve, etc.
Construction Defects: dome cracking
Others: flooding, not feeding, fire, lack of space, sickness, etc.
Figure 18: Reason for digester non-functionality.24
0
10
20
30
40
50
60
70
80
90
100
P e r c e n
t
Till 1990 1990-94 1994-96 1996-98
Year of Installation
Functional Non-Functional
Figure 19: Functionality rate and age of digesters.25
Public Health: The lack of oxygen in a biogas digester that results anaerobic digestion also
means many other micro-organisms are not able to survive. There are a few pathogens that
survive (e.g. Ascaris lumbricoides, ringworm), but most have a high die-off rate (Figure 20). The
National Academy of Sciences report on biogas generation from human and animal wastes
concluded that anaerobic digestion is most effective practical treatment of human excreta.26
Because of the harsh conditions in the slurry product, die-off continues outside of digester.
98.589
99 10090
100
0
20
40
60
80
100
D i e - o f f , %
P o l i o v
i r u s
S a l m o n
e l l a
s s p
.
S a l m o n
e l l a
t y p h o
s a
M y c o b a c t e r i u m
t u b e r c u l o s i s
A s c
a r i s
P a r a s i t e c y s t s
Figure 20: Pathogen die-off during anaerobic
digestion.27
1.8695
0.401
1.8515
0.014
0
1
2
3
g N / k g d u n g
Fresh slurry Evaporated slurry
Organic N Ammonia N
Figure 21: Nitrogen retention in biogas digester
slurry.28
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0
1
2
3
4
5
6
7
7 14 23 30 43 60
Digestion period (days)
B i o g a s p r o d
u c e d ( l i t e r s )
0% W.H.
30% W.H.
50% W.H.70% W.H.
100% W.H.
Figure 24: Effect of substitution of water hyacinth for cattle manure on biogas production.33
(Total feedstock mass =500 g)
Other benefits: Other important benefits of biogas energy include the following:
• It is a clean burning fuel (manure combustion is especially sooty).
• It allows time savings in fuel collection and cooking. One study showed savings of at
least 1 hour in households using biogas.
• The fuel is easy to store and use.
• It results in reduced transfer of fungus and other pathogens to next year’s crop.
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Biogas as a Residential Energy Source for South Asia Daniel Thomas, 16
References
1AskAsia Resources. Maps. www.askasia.org/image/maps/asias1.htm 12-13-2003.
2Johnstone, Patrick, and Mandryk, Jason, “Operation World,” WEC International: Bulstrode, 2001.
3Energy Information Agency, “International Total Primary Energy and Related Information,” Table E1: World
Total Primary Energy Consumption, www.eia.doe.gov/emeu/iea/tablee1.html 12-13-2001.4Ibid.
5Ibid.
6United Nations Statistics Division, 1998 Energy Balances and Electricity Profiles,
http://unstats.un.org/unsd/energy/balance/tables.htm7 Ibid.8
National Academy of Sciences. “Methane Generation from Human, Animal, and Agricultural Wastes.” NAS:
Washington, D.C., 1977. p 6.9
National Academy of Sciences, p 41.10
Moller, H. B., Sommer, S. G., Ahring, B. K. “Methane productivity of manure, straw and solid fractions of
manure.” Biomass and Bioenergy. July 2003. p 7.11
Ibid.12
Moller, H. B., et al. p 6.13
National Academy of Sciences, p 69.14U.S. Congress, Office of Technology Assessment, “Fueling Development: Energy Technologies for Developing
Countries,” OTA-E-516 Washington, DC: U.S. Government Printing Office, April 1992.15
Hougen, O. A., Watson, K. M., Ragatz, R. A., “Chemical Process Principles: Part I, Material and Energy
Balances.” 2nd
Ed. New York: Wiley, 1954. p 401.16
Tillman, D. A. “The Combustion of Solid Fuels and Wastes.” San Diego: Academic P., 1991. p 87.17
National Academy of Sciences, p 45.18
Gatade, S. P., Rao, B. H. “All India Seminar on Renewable Sources of Energy.” Burning Characteristics of
Gobar Gas. Allahabad, India: Eastern P, 1982. p B-5.119
Tillman. p 37.20
Pande, B. M. “Performance of Bio-Gas Plants: A Field Study.” Lucknow, India: Appropriate Technology
Development Association, 1985. p 49.21
Ibid.22
National Academy of Sciences, p 121.23 Dutta, S., Rehman, I. H., Malhotra, P, Venkata, R. P. “Biogas: the Indian NGO Experience.” New Delhi: Tata
Energy Research Institute, 1997. p 16.24
Ibid.25
Ibid. p 1726
National Academy of Sciences, p 56.27
Ibid. p 55.28
Ibid. p 50.29
Chau, Le Ha. “Biodigester effluent versus manure from pigs or cattle as fertilizer for production of cassava
foliage.” Livestock Research of Rural Development. Vol 10. No. 3, 1998.
<www.cipav.org.co/lrrd/lrrd10/3/chau1.htm>30
Singh, S., Kumar, M. “All India Seminar on Renewable Sources of Energy.” Utility of Eichornia Crassipes as a
Source of Bio Gas. Allahabad, India: Eastern P, 1982. p B-3.331
Chau, Le Ha.
32 Ibid.33
Singh, S., Kumar, M. p B-3.1